It has been widely believed that this is due to the ‘antioxidant’ behaviour of the vitamins and phytochemicals present in these foods. However, recent research into phytochemicals has indicated that there are mechanisms other than antioxidant behaviour involved.
Antioxidants – keeping harmful ‘free radicals’ in check
Oxidising aluminium (gain of oxygen)
2Al(s) + 3O2(g) → Al2O3(s)
Oxidation of ethanol (loss of hydrogen)
C2H5OH(l) → CH3CHO(g)
Rusting of iron (loss of two electrons)
Fe(s) + moist air → Fe2+(aq) + 2e
There are a number of biological processes involving oxidation that are of benefit to the body, such as the release of energy from foods, for example:
However, oxidation can result in the production of highly reactive particles known as ‘free radicals’. Free radicals have important signalling functions in the body. For example, the inner lining of blood vessels uses the nitric oxide radical to signal the surrounding smooth muscle to relax. This results in an increased blood flow in that area.
In excess, free radicals cause damage to various cell structures, particularly cell membranes and the internal contents of the cell, especially DNA in the nucleus and in small organelles present in cells known as mitochondria. Over time, this damage is cumulative, and there is an increasing body of scientific evidence to link excess free radicals within the body to cardiovascular disease, stroke, diabetes, cancer and acceleration of the general ageing process.
An antioxidant is a chemical substance that prevents oxidation by chemical means. The body is capable of producing its own antioxidants but there is a delicate balance between the advantageous and detrimental effects of free radicals, and lifestyle choices can have a major impact on this.
Vitamins as antioxidants
Claims have been made about the antioxidant capabilities of certain vitamins. It would appear that:
- vitamin A plays an important role in the retina of the eye by allowing the neural transmission of light into vision – claims have been made about its antioxidant capability, but the evidence backing this up is inconclusive
- vitamin C has many functions within the body, and its antioxidant role has been well established – its water solubility allows it to act effectively against waterborne free radicals
- vitamin E is fat soluble and its main function is to inhibit oxidation of fatty acids found in the bloodstream and as part of cell membranes – it can be regenerated in the body in a process that uses vitamin C.
Some people believe that supplementing their diet with these vitamins will be beneficial, but a balanced diet rich in fruits and vegetables has been found to provide all of the necessary vitamins normally needed by the body.
Phytochemicals as antioxidants
Phytochemicals are biologically active compounds found in relatively small amounts in plant foods. Some of these compounds have a protective function within the plant as well as being responsible for certain plant colours. For example, cyanidin is a phytochemical belonging to the polyphenol group. It has a purple-red colour and is found in the skins of red coloured fruits and vegetables.
It was thought that polyphenolic compounds present in the fruit and vegetables we eat had an antioxidant effect within the body, but recent research conducted by scientists at Plant & Food Research in collaboration with overseas scientists has revealed that this effect has been overstated.
The new theory is that the polyphenolic compounds actually stimulate the body’s own antioxidant defences rather than being the actual antioxidant. They are mildly toxic in excess and can be likened to a vaccine stimulating the immune system – when we do eat them in small amounts, it appears that the low dose of toxin results in the body regulating its antioxidant capabilities.
One theory put forward to explain this is that of hormesis. Hormesis is the term for favourable biological responses to low exposures to toxins and other stressors. A toxin showing hormesis has the opposite effect in small doses as in large doses.